The present invention relates to composite metal materials. More particularly, the present invention relates to composite metal materials having high electrical conductivity, controlled thermal properties and excellent mechanical properties at high temperatures.
Various applications require metals having high electrical conductivity and high strength at elevated temperatures. Such properties are typically not available from a single metal.
Generally, it is difficult to significantly modify the physical properties of alloy metals by adjusting the proportions of the alloy constituents. Composite metal materials, such as composite metal laminates, have been used to provide unique combinations of properties that can be obtained by cladding dissimilar core and clad materials. The resultant materials provide the ability to vary the electrical, mechanical and thermal properties.
Electrically conductive spring materials and members used in making electrically conductive spring contacts in switches and in sockets for mounting integrated circuits on printed circuit boards are examples of applications requiring high electrical conductivity and high strength at elevated temperatures. Other applications for materials having high electrical conductivity and high strength are lead frames for semiconductors. In many applications, electrical connectors are used in extremely high temperature environments. For example, automobile engine compartments include electrical connectors that are exposed to extreme high and low temperatures. Therefore, it is desirable to provide an electrical connector that exhibits improved resistance to stress relaxation at high temperatures.
A variety of materials are used to manufacture electrically conductive springs and connectors. Examples of such material include beryllium copper and copper clad stainless steels. Beryllium copper alloys are used in many applications requiring electrical conductivity and mechanical reliability at high temperatures.
Due to potential concerns about adverse health and environmental effects associated with beryllium and beryllium copper use, it would be desirable to provide a material that did not contain beryllium but still had the desirable electrical and mechanical properties of beryllium copper alloys. However, research has yielded few materials that exhibit the combined properties of electrical conductivity and mechanical reliability over a wide temperature range. Therefore, it would also be desirable to provide a composite material having electrical, thermal and mechanical properties that could be tuned or adjusted over a wide temperature range. It would be particularly advantageous if the properties could be tuned or adjusted by varying the relative proportions of the materials in the composite material and/or by heat treating the material to provide a material that has high mechanical strength and capable of being manipulated into complex shapes.
Accordingly, the present invention generally provides a composite material including a core of precipitation hardenable core and a clad including a transition metal or transition metal alloy. As used herein, the term core is not limited to mean that the core is completely surrounded by cladding material. Instead, the term core is used in a broader sense to mean the innermost part of the composite material, and at least one surface of the core is covered by cladding material. For example, in a situation in which a planar composite material is provided, at least two major surfaces of the plane would be covered with cladding material.
Preferably, the precipitation hardenable core material has a coefficient of thermal expansion (CTE) less than about 9 parts per million per xc2x0 C. (ppm/xc2x0 C.) over the temperature range of 20xc2x0 C. to 100xc2x0 C. According to one aspect of the invention, the core metal includes 32 to 50 percent by weight of Ni, 1.5 to 3.5 percent by weight of Ti, and 0.05 to 1.0 percent by weight of Al. According to this aspect, the balance of the material includes Fe. Preferably, the core metal includes 35 to 45 percent by weight of Ni, 2 to 3 percent by weight of Ti, and the balance Fe. The core material may also include trace impurities including, but not limited to Mn, Si, C, S, and P. Preferably, each of the trace impurities is present in amount less than one weight percent.
According to another aspect of the invention, the cladding layer metal has a coefficient of thermal expansion greater than 9 ppm/xc2x0 C. in the temperature range of 20xc2x0 C. to 100xc2x0 C. The cladding material preferably includes a metal selected from the group consisting of copper, nickel, zinc and alloys thereof.
In another aspect of the invention, the core material comprises 50% to 90% by volume of the composite material. Preferably, the core material comprises 70% to 90% by volume of the composite material. The coefficient of thermal expansion of the composite is preferably less than about 9 ppm/xc2x0 C. in the temperature range of 20xc2x0 C. to 100xc2x0 C. In addition, according to another aspect, the ratio of the 0.2% offset yield strength to tensile strength of the composite after heat treatment at 750xc2x0 C. from the annealed condition is less than 0.85.
Another aspect of the invention relates to a method of making a composite material including providing a core of the precipitation hardenable metal described above and at least one layer of a transition metal or transition metal alloy cladding covering at least one surface of the core and roll bonding the core layer and the cladding layer together to form the composite material. Another aspect of the method may further include of annealing the composite material at a temperature between 850xc2x0 C. and 1000xc2x0 C. According to another aspect, the invention may include heat treating the composite material at a temperature of 700xc2x0 C. to 800xc2x0 C. for at least one-half hour. In a preferred aspect of the invention, the heat treatment is performed so that a gamma phase material is formed during said heat treatment. Preferably, the heat treatment is performed so that the ratio of the 0.2% offset yield strength to tensile strength of the composite after heat treatment from the annealed condition is less than 0.85.
In another aspect, the roll bonding step is performed in a single pass with a thickness reduction of at least 30% and the clad layers comprise at least 20% of the composite by volume. According to this aspect, the core and clad materials are planar, and an equivalent volume of clad material covers both major surfaces of the core.
One advantage of the present invention is that the composite material and the method of making the composite material provide a material that has adjustable or tunable electrical, thermal and mechanical properties. The precipitation hardenable core enables the strength properties of the composite material to be adjusted by heat treating the material. The electrical and thermal properties of the composite material can be adjusted by varying the ratio of the cladding material to the core material.
Additional features and advantages of the invention will be set forth in the description which follows. It is to be understood that both the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the invention as claimed.
The present invention relates to the discovery of a unique combination of materials to provide a composite having tunable electrical, thermal and mechanical properties. Advantageously, the properties of the material can be tuned by adjusting the volume ratio of the core and cladding materials that form the composite. Alternatively, the properties of the composite can be adjusted by heat treating the material. Of course, the properties of the composite can be optimized by adjusting the relative amounts of the core and cladding materials and by heat treating the composite.
The core material includes a precipitation hardenable material having a coefficient of thermal expansion less than about 9 ppm/xc2x0 C. over the temperature range of about 20xc2x0 C. to about 100xc2x0 C. The cladding material includes a transition metal element or an alloy of transition elements, such as Cu, Ni, Zn, Au, and Ag. The cladding material has a coefficient of thermal expansion preferably greater than about 9 ppm/xc2x0 C.
As used herein, the terms precipitation hardenable and precipitation hardening broadly refers to altering the strength and hardness of materials, particularly metal materials, by heat treating the materials. More specifically, as is known in the art, precipitation hardening of a material involves heat treating a material containing more than one element so that fine dispersed particles of a second phase form during heat treatment, which usually increases the strength and hardness of the material.
An example of a precipitation hardenable material that is suitable for use as a core material in the present invention is an iron alloy material containing nickel and two or more elements capable of forming a second phase during heat treatment along with trace levels of impurities. Examples of impurities include, but are not limited to manganese, silicon, carbon, sulfur and phosphorus. When an iron alloy of this type comprising nickel, titanium and aluminum is precipitation hardened by heat treatment, a gamma (xcex3) phase material including nickel, titanium and aluminum is formed. Preferably, the core metal contains 32 to 50 percent by weight of Ni, 1.5 to 3.5 percent by weight of Ti, 0.05 to 1.0 percent by weight of Al, and the balance of the material including Fe. More preferably, the core metal includes 35 to 45 percent by weight of Ni, 2 to 3 percent by weight of Ti, less than 1.0 percent by weight of Al, and the balance Fe. The core material may also include trace impurities including, but not limited to Mn, Si, C, S, and P. An example of such a commercially available precipitation hardenable material is Gammaphy(copyright), which is manufactured and sold by the assignee of the present invention.
According to the present invention, the precipitation hardenable core material is clad with a transition metal or transition metal alloy. Particularly suitable clad materials are nickel and copper and alloys containing nickel and copper. Preferably, the core material comprises at least about 50% by volume of the composite core clad material, and more preferably, the core material comprises between about 70% and 90% by volume of the composite material. Cladding of the transition metal material can be accomplished by any suitable method known in the art such as roll bonding.
The invention is further illustrated by the following examples, which are intended to be illustrative, and not in any way limiting, to the claimed invention. The electrical and mechanical properties of the composite materials were compared with beryllium copper alloys used for a variety of applications, such as, for example, electrical connectors.
In Table I, alloy 172 is commercially available beryllium copper alloy number C17200 and alloy 175 is commercially available beryllium copper alloy number C17500. Beryllium copper materials are available in a wide variety of compositions, each of which provides a specific set of properties for a given temperature condition. In the column labeled xe2x80x9ctemper,xe2x80x9d xe2x80x9cAxe2x80x9d refers to an alloy that is formed from cold rolling and annealed and xe2x80x9cATxe2x80x9d refers to an alloy heat treated from the annealed condition.
Although beryllium copper alloys can be heat treated from cold roll condition, to provide desirable mechanical properties, these alloys as mill hardened generally exhibit poor formability.
In Table II, the properties of Gammaphy(copyright) are provided for comparison with the properties of the examples in which Gammaphy(copyright) is clad with transitional metal. Gammaphy(copyright) is commercially available from the assignee of the present invention. The Gammaphy used to obtain the information in Table II included 42.3% Ni, 0.2% Al, 2.6% Ti, and impurities including less than 1% Mn, less than 1% Si, less than 0.2% Al, less than 0.1% C, less than 0.05% S, and less than 0.05% P, with the balance of the material comprising Fe. The percentages of the constituents are in weight percent. The electrical conductivity of the Gammaphy(copyright) sample was 2.2% IACS and the coefficient of thermal expansion over the temperature range of 20xc2x0 C. to 100xc2x0 C. was 3.9 ppm/xc2x0 C.